1. Field of the Invention
The present invention relates to electronic circuits and, more particularly, to electronic circuits for frequency to current conversion.
2. Description of Related Art
Frequency-to-voltage and frequency-to-current converters can be employed in numerous types of applications. In particular, frequency-to-current converters will be important building blocks that are widely used in a variety of applications. Two such applications are phase-locked-loops (PLLs) and analog-to-digital converters (ADCs). A PLL is an electronic circuit that controls an oscillator so that it maintains a constant phase angle on the frequency of an input, or reference, signal. A PLL ensures that a signal is locked on a specific frequency and phase and can also be used to generate, modulate and demodulate a signal or multiply a frequency. An ADC is a device that converts analog signals into digital signals. Applications such as ADCs, can utilize features such as an adaptive bias current, which enable analog-to-digital conversion while saving power consumption. Tools such as a frequency-to-current converter may be employed in such applications to supply an adaptive bias current.
Standard implementations of frequency-to-current converters, however, are inadequate to such tasks. Frequency-to-current converters are often implemented by coupling a frequency-to-voltage converter to a voltage-to-current converter. Many conventional frequency-to-voltage and voltage-to-current converters are well known in the art. This combination of circuits, however, is often inadequate because such combinations are complicated to be embedded in a single integrated circuit, and demand a very large silicon area. Circuits such as these in many instances also require the use of a buffer to avoid disturbing the reference frequency.
Some other frequency-to-current converters require the counting of the number of pulses over a fixed period of time. Additional designs require low pass filtering fixed duration pulses at a rate set by an input frequency. Designs such as these are unsuitable for low frequency applications because they produce an output that is usually affected by AC ripple components and also uses a time-consuming averaging process. Furthermore, such designs are based on complex circuits that consume a large amount of power.
Another current frequency-to-current converter design is based on a non-linear analog circuit where the input signal frequency information is extracted through a differentiator and an integrator. However, although such a circuit has a fast response time to input frequency changes, it demands extremely precise differentiators and integrators. A phase mismatch between the differentiator and the integrator can create a large spike at the output.
Accordingly, a need exists for a frequency-to-current converter with reduced complexity, a fast start-up time, a very small area requiring little silicon for integration and low AC ripple output current. A further need exists for a frequency-to-current converter that can be easily incorporated into integrated CMOS mixed signal applications such as PLLs and ADC's.